Encapsulation can be broadly defined as the formation of small, spherical particles that incorporate an active agent. The first commercial application of encapsulation was by the National Cash Register Company, who developed an improved copying paper using two dyes that were coated with a clay. When these capsules were ruptured by the application of pressure, a colored imprint was produced. This successful application triggered other uses in agriculture, pharmaceuticals, oil industries, food industries, and consumer products .
Because such spherical particles are very small, usually in the range of several to about 20 microns, the process of forming such particles is referred to as microencapsula-tion. However, we need to distinguish between microcapsules and microspheres. Microcapsules have a core containing the active agent surrounded by a membrane, whereas microspheres are solid particles that contain an active agent homogeneously dispersed within the solid matrix. Microspheres can be either solid or porous. These three types are shown schematically in Figure 1.
Release of agents incorporated into microcapsules can occur either abruptly, as in the National Cash Register Company product, or the ''scratch and sniff'' product manufactured by the 3M Company, where the outer membrane is ruptured by the application of pressure or can occur in a controlled manner by diffusion of the active agent from the core through the outer rate-limiting membrane. In the latter case, if the thermodynamic activity of the drug in the core reemains constant and the drug is removed rapidly from the aqueous environment surrounding the microcapsule, constant release kinetics, referred to as zero order, are obtained. No such products have been applied to the cosmetics and cosmeceutical field, but have been extensively investigated in controlled-release applications, particularly in contraception  and narcotic addiction .
Agents incorporated into microspheres are released by kinetics that are typical of matrix systems and follow t1/2 kinetics as predicted by the Higuchi equation . Thus, initial release rate is rapid and then declines as the thickness of the drug-depleted layer increases. Studies of release kinetics from biodegradable porous microspheres indicate that release kinetics similar to that noted for matrix-type microspheres are obtained .
Other than liposomes, which are covered in Chapter 17, only one type of micro-particulate has found important applications in cosmetics and skincare technology, and these are porous microspheres. This chapter will cover the application of porous microspheres in cosmetics and skincare applications.
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